1. Field of the Invention
The present invention relates to a method for controlling a shape measuring apparatus.
2. Description of Related Art
There is known a shape measuring apparatus which measures a shape of an object to be measured by moving a stylus tip along a surface of the object to be measured while scanning the surface (for example, JP 2008-241420 A, JP 2013-238573 A, JP 2014-21004 A).
To perform scanning measurement, a path for the scanning measurement needs to be prepared. The apparatus disclosed in JP 2008-241420 A converts design data based on CAD data (for example, non-uniform rational B-spline (NURBS) data) into a group of polynomials curves in a predetermined degree.
This procedure is briefly described below.
First, CAD data (for example, NURBS data) including path information is received from an external CAD system and converted into data of a group of points. The data of each point is a combination of coordinates (x,y,z) and normal line directions (P,Q,R) (that is, (x,y,z,P,Q,R)). Hereinafter, the data of the group of points including the information (x,y,z,P,Q,R) is referred to as “contour point data” for the sake of the description below.
Next, the coordinates of each point are offset in the normal line direction by a predetermined amount. (The predetermined amount is, specifically, a stylus tip radius r−a reference amount of deflection E0). The data of the group of points data obtained in this manner is referred to as “offset contour point data”.
Then, the offset contour point data is converted into a group of polynomial curves in a predetermined degree.
Here, it is assumed that the polynomial is a cubic function, and the curves are parametric cubic curves (PCC). A path for measuring a workpiece is generated based on the PCC curve. Furthermore, the PCC curve is divided into a group of divided PCC curves.
By calculating a speed curve from the group of divided PCC curves, a moving speed (movement vector) of a probe is calculated.
(For example, the moving speed (movement vector) of the probe is set based on the curvature of each segment of the group of divided PCC curves or the like.)
By moving the probe based on the moving speed calculated in this manner, the stylus tip is moved while scanning the surface of the object to be measured (passive nominal scanning measurement: note that the word “nominal” in this description means scanning along a predetermined trajectory calculated in advance based on design data of an object).
Furthermore, there is known a method for performing scanning measurement while correcting a trajectory by continuously calculating a deflection correcting vector so as to keep an amount of deflection of a probe to be constant (JP 2013-238573 A).
Hereinafter, such nominal scanning is referred to as active nominal scanning measurement.
The active nominal scanning measurement disclosed in JP 2013-238573 A is briefly described.
In the active nominal scanning measurement, a combined velocity vector V represented by the following Expression 1 is a movement instruction for a probe.
The probe is moved based on the combined velocity vector V, and thereby scanning measurement to a workpiece surface in which the probe (stylus tip) moves along a PCC curve and an amount of deflection is constant, that is, active nominal scanning measurement is implemented.V=Gf×Vf+Ge×Ve+Gc×Vc  (Expression 1)
With reference to FIG. 1, Expression 1 is briefly described.
In FIG. 1, there is a PCC curve (that is, a scanning path) at the position offset from the design data (contour point data) by a predetermined amount (a stylus tip radius r−a reference amount of deflection E0). Furthermore, in FIG. 1, the actual workpiece is slightly shifted from the design data.
The vector Vf is a path velocity vector. The path velocity vector Vf has a direction from an interpolation point (i) on the PCC curve to the next interpolation point (i+1). Note that, the magnitude of the path velocity vector Vf is determined based on, for example, the curvature of the PCC curve at the interpolation point (i) (for example, JP 2014-21004 A).
The vector Ve is a deflection correcting vector to keep the amount of deflection Ep of the probe to be a predetermined reference amount of deflection E0 (for example, 0.3 mm).
(The deflection correcting vector Ve is necessarily to be parallel to the normal line of the workpiece surface.)
The vector Vc is a trajectory correcting vector. The trajectory correcting vector Vc is parallel to a perpendicular from the probe position to the PCC curve.
In Expression 1, Gf, Ge, and Gc are a scanning driving gain, a deflection correcting gain, and a trajectory correcting gain respectively.